This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-074683, filed Mar. 15, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a sum frequency light generation method and a sum frequency light generation apparatus adaptable for wide-band wavelength conversion and more particularly to a sum frequency light generation method and a sum frequency light generation apparatus for obtaining an angular frequency by summing up angular frequencies possessed by two lights projected from a pair of separate light sources.
2. Description of the Related Art
In recent years, laser lights having various kinds of angular frequencies (wavelength) depending on application purpose have been employed in respective technical fields of high density memory, optical display light source, medical instrument, color printer and the like.
The wavelength range of laser light for use has been expanding from visible lights of red, orange, green, blue and the like to ultraviolet bands.
The lights having these respective wavelengths can be generated by means of each dedicated laser light source.
However, generally, a method of converting laser light to each wavelength through a nonlinear optical crystal has been actually employed because this system can be realized at a low cost with a relatively simple configuration.
As a technology for converting the wavelength through this nonlinear optical crystal, since before, a method in which second harmonic generation is generated through a nonlinear optical crystal (hereinafter referred to as type 1 phase matching crystal) capable of matching the phase (type 1) and converted has been employed.
According to this method, basically, the angular frequency of inputted laser light is multiplied by 2 through the type 1 phase matching crystal. A first light having an angular frequency xcfx891 and a second light having an angular frequency xcfx892, whose polarization directions are parallel to each other, are multiplexed through a type 1 phase matching crystal so as to convert to a light having a twofold angular frequency 2xcfx891.
However, according to the wavelength conversion method through the type 1 phase matching crystal, if it is intended to convert two lights having each different frequency to a sum frequency generation light (SFG light) having a sum frequency of these frequencies, the first light having the angular frequency light xcfx891 and the second light having the angular frequency xcfx892, whose polarization directions are parallel to each other, are inputted to the type 1 phase matching crystal.
Thus, actually, three kinds of lights each having an angular frequency 2xcfx891, 2xcfx892, xcfx891+xcfx892 are produced.
Therefore, if the angular frequencies xcfx891 and xcfx892 are near each other, separation of two lights having angular frequencies 2xcfx891 and 2xcfx892 and sum frequency generation light having an angular frequency xcfx891+xcfx892 becomes difficult, so that it is impossible to pick up a desired light alone, which is a problem to be solved.
Further, if looking in terms of efficiency, actual conversion efficiency for converting to the sum frequency generation light having the angular frequency xcfx891+xcfx892 drops because excessive two lights having the angular frequencies 2xcfx891 and 2xcfx892 are outputted, which is another problem to be solved.
That is, the method and apparatus for generating the sum frequency generation light by wavelength conversion using the type 1 phase matching have such problems.
Thus, to solve this problem, a method of generating the sum frequency light generation using a nonlinear optical crystal (hereinafter type 2 phase matching crystal) capable of executing type 2 phase matching has been invented.
Here, the wavelength conversion based on the type 2 phase matching will be described in detail.
As described above, the wavelength conversion by type 1 phase matching multiplexes the first light having the angular frequency xcfx891 and the second light having the angular frequency xcfx891, with the polarization directions thereof being parallel to each other, so as to produce the light having the angular frequency 2xcfx891 which is twofold.
Contrary to this, the wavelength conversion by type 2 phase matching multiplexes a first light having an angular frequency xcfx891 and a second light having an angular frequency xcfx892, with the polarization direction thereof being perpendicular to each other, so as to produce a light having an angular frequency xcfx891+xcfx892.
Because the wavelength conversion by the type 2 phase matching is not carried out but between lights having polarization directions perpendicular to each other, even when the first light having the angular frequency xcfx891 and the second light having the angular frequency xcfx892 are entered, different from the wavelength conversion by the type 1 phase matching, only the light having the angular frequency xcfx891+xcfx892 which is the SFG light is outputted to outside as wavelength converted light.
Further, because the wavelength conversion by the type 2 phase matching does not produce the light having the angular frequency 2xcfx891 or the light having the angular frequency 2xcfx892, efficiency of conversion to the SFG light having the angular frequency xcfx891+xcfx892 is excellent.
Thus, the wavelength conversion method which generates the SFG light using the type 2 phase matching crystal may be used for the purpose of obtaining a correlation signal between two optical signals and in recent years, an SFG light generating apparatus for that purpose has been developed.
FIG. 9 is a schematic structure diagram of a concrete apparatus based on the sum frequency light generation method using the type 2 phase matching crystal.
A first light xe2x80x9caxe2x80x9d entered to an input terminal 1 from outside having a wavelength xcex1 (angular frequency xcfx89D) and linearly polarized, is controlled in terms of its polarization direction by a polarization direction controller 2, so as to be directed at 90xc2x0 with respect to a reference direction (0xc2x0) and after that, entered to a multiplexer 3.
On the other hand, a second light xe2x80x9cbxe2x80x9d entered to an input terminal 4 from outside, having a wavelength xcex2 (angular frequency xcfx89S) and linearly polarized, is controlled in terms of its polarization direction by a polarization direction controller 5, so as to be directed to, for example, the reference direction (0xc2x0) and after that, entered to the multiplexer 3.
The multiplexer 3 composed of for example, a beam splitter and the like, reflects the first light xe2x80x9caxe2x80x9d at right angle with a half mirror 3a while allowing the second light xe2x80x9cbxe2x80x9d to advance straight.
Thus, this multiplexer 3 multiplexes the entered first light xe2x80x9caxe2x80x9d and the second light xe2x80x9cbxe2x80x9d with the polarization directions thereof being perpendicular to each other on the same optical axis.
The first light xe2x80x9caxe2x80x9d and the second light xe2x80x9cbxe2x80x9d emitted from this multiplexer 3 with the polarization directions thereof being perpendicular to each other are entered to one face of the nonlinear optical material 6 which is cut to match with a phase matching direction corresponding to a purpose from the type 2 phase matching crystal.
This nonlinear optical material 6 emits a sum frequency light xe2x80x9ccxe2x80x9d having an angular frequency xcfx89D+xcfx89D (wavelength xcex3) which is a sum of the angular frequencies xcfx89D and xcfx89S possessed by each of the first light xe2x80x9caxe2x80x9d and the second light xe2x80x9cbxe2x80x9d with the polarization directions thereof being perpendicular to each other, to an output terminal 7 from the other face.
The phase matching direction will be explained in detail hereinafter.
For the sum frequency light to be generated, the velocity (phase velocity) of each incident light within crystal of a nonlinear optical material 6 needs to coincide with the velocity (phase velocity) of that sum frequency light within the crystal.
Such a direction in which these phase velocities coincide with each other most within the crystal of the nonlinear optical material 6 is called phase matching direction and if light is propagated in this direction, efficiency of the wavelength conversion is the maximum.
The phase matching direction will be described further with a refractivity ellipsoid 9 shown in FIG. 4A.
For example, a direction 12 of connecting an intersection 11 between a refractivity ellipsoid 9 about the first light having the angular frequency xcfx89D, a refractivity ellipsoid 9 about the second light having the angular frequency xcfx89S and a refractivity ellipsoid 10 about the sum frequency light having the angular frequency (xcfx89D+xcfx89S) with the origin of the refractivity ellipsoids 9 and 10 is the phase matching direction.
In FIG. 4A, for simplification of a description, it is assumed that the refractivity ellipsoid about the second light having the angular frequency xcfx89S is equal to the refractivity ellipsoid about the first light having the angular frequency xcfx89D.
An angle xcfx86 formed between a direction in which the phase matching direction 12 is projected to a plane containing axis xe2x80x9caxe2x80x9d and axis xe2x80x9cbxe2x80x9d and the axis xe2x80x9caxe2x80x9d of the crystal and an angle xcex8 formed between the phase matching direction 12 and the axis xe2x80x9ccxe2x80x9d of the crystal as shown in FIG. 4B are called phase matching angle.
Generally, the nonlinear optical crystal allowing type 2 phase matching is cut out in the form of a rectangular solid or cylinder containing a plane 13 intersecting the aforementioned phase matching direction 12 and used such that the propagation direction of light coincides with the phase matching direction 12.
In this case, an angle (cutout angle) relative to the axis (axis xe2x80x9caxe2x80x9d, axis xe2x80x9cbxe2x80x9d, axis xe2x80x9ccxe2x80x9d) of the crystal when the nonlinear optical crystal is cut out is determined depending on the aforementioned phase matching angles xcfx86 and xcex8.
Because the refractivity ellipsoid differs depending upon the kind of the nonlinear optical crystal, wavelength of light propagating through the crystal and the polarization direction of the light relative to the axis of the nonlinear optical crystal, if these are determined, the aforementioned cutout angle is automatically determined.
For example, in case of 2-adamantylamino-5-nitropyridine (AANP), which is an organic nonlinear optical crystal, it has been well known that the phase matching angles xcfx86 and xcex8 are 90xc2x0 and 60xc2x0 respectively when the wavelength of the first and second lights is 1.55 xcexcm and the phase matching angles xcfx86 and xcex8 are 60xc2x0 and 90xc2x0, respectively when the wavelength of the first and second lights is 1.3 xcexcm.
That is, because the crystal cut out for use under a wavelength of 1.55 xcexcm has an angle xcex8 of 90xc2x0 as described above, the plane 13 intersecting the phase matching direction 12 is a plane parallel to the axis a.
In case of the SFG, if when the axis xe2x80x9caxe2x80x9d is regarded as the reference axis, light having a polarization direction parallel to the reference axis and light having a polarization direction vertical to that reference axis are multiplexed and entered through such a crystal, the SFG light having a polarization direction parallel to the reference axis is obtained.
Because the crystal cut out for use under a wavelength of 1.3 xcexcm has an angle xcex8 of 90xc2x0, the plane 13 intersecting the phase matching direction 12 is a plane parallel to the axis c.
In case of the SFG, if when the axis xe2x80x9ccxe2x80x9d is regarded as the reference axis, light having a polarization direction parallel to the reference axis and light having a polarization direction vertical to that reference axis are multiplexed and entered through such a crystal, the SFG light having a polarization direction parallel to the reference axis is obtained.
Meanwhile, a specification of an ordinarily marketed nonlinear optical crystal has a description about the aforementioned reference axis.
Therefore, a technology for converting the entered first light xe2x80x9caxe2x80x9d and second light xe2x80x9cbxe2x80x9d through the nonlinear optical crystal to light having a sum angular frequency of the respective angular frequencies can be a method of obtaining a sum frequency light effectively and easily by using the type 2 phase matching crystal as the nonlinear optical crystal as described above.
If under this method, the nonlinear optical material 6 corresponding to an application wavelength is employed so that an optical system (optical positional relation) which secures an incident angle corresponding to the nonlinear optical material 6 is prepared, various kinds of wavelengths can be obtained.
In recent years, a technology about conversion to a target wavelength in wide band without changing optical positional relation between respective polarization direction controllers 2 and 5, multiplexer 3 and nonlinear optical material 6 in the sum frequency light generating apparatus as shown in FIG. 9 has been demanded in a measuring instrument in communication filed.
Thesis ECOC ""96 ThB1.2 and the like have stated that changing the wavelength of light while fixing the wavelength of the other inputted light by using KTP or AANP (2-admantyl 5-nitorpyrdine), which is a type 2 phase matching crystal, makes it possible to change a sum frequency light generation region to a wide band.
This thesis indicates conversion efficiency of the sum frequency light c outputted when the wavelength of one light is fixed to 1555 nm while the wavelength of the other light is changed between 1530 nm and 1580 nm.
The band width in which the obtained SFG light is generated is 35 nm in case of KTP if it is defined in the width of 3 dB and in case of AANP, it is about 40 nm.
However the conventional sum frequency light generation method and sum frequency light generating apparatus using the type 2 phase matching crystal described above has still following problems to be solved.
That is, as communication region in recent wavelength division multiplexing (WDM) transmission expands to 80 nm or more, SFG light generation band of 80 nm or more has been demanded in communication field of the measuring instrument.
However, the conventional sum frequency light generation method and apparatus just employing the type 2 phase matching crystal cannot convert the wavelength of 80 nm or more unless the nonlinear optical material 6 is replaced corresponding to a target wavelength or the relation in position of its optical components is changed.
As described above, if the band width in which a realized SFG light is generated is 35 nm in case of KTP and 40 nm in case of AANP if it is defined in the width of 3 dB, when the wavelength of one light is fixed to 1555 nm while the wavelength of the other light is changed between 1530 nm and 1580 nm.
Accordingly, the present invention has been achieved in views of the above-described problem and therefore, an object of the present invention is to provide a sum frequency light generation method and a sum frequency light generation apparatus using type 2 phase matching crystal, capable of expanding its wavelength conversion bandwidth twice or more a conventional example.
Another object of the present invention is to provide a sum frequency light generation method and a sum frequency light generation apparatus capable of setting the center of its wavelength conversion band arbitrarily so as to meet wavelength conversion in a wider band.
To achieve the above described object, according to a first aspect of the present invention, there is provided a sum frequency light generation method comprising: receiving a fixed wavelength light (b1) having a single polarization direction and a fixed wavelength (xcex2) to control the polarization direction of the fixed wavelength light; receiving a variable wavelength light (a1) having a single polarization direction and a variable wavelength (xcex1) to control the polarization direction of the variable wavelength light so as to be perpendicular to the polarization direction of the fixed wavelength light; by entering the fixed wavelength light whose polarization direction is controlled and the variable wavelength light whose polarization direction is controlled into a nonlinear optical crystal (8) allowing type 2 phase matching, multiplexing the fixed wavelength light having an angular frequency xcfx89D and the variable wavelength light having an angular frequency xcfx89S, with polarization directions thereof being perpendicular to each other, through the nonlinear optical crystal so as to produce a sum frequency light having an angular frequency xcfx89D+xcfx89S; and when multiplexing the fixed wavelength light and the variable wavelength light through the nonlinear optical crystal, controlling the polarization direction of the fixed wavelength so as to be parallel to a predetermined reference axis within a plane perpendicular to a phase matching direction (12) of the nonlinear optical crystal, wherein the predetermined reference axis is a single axis parallel to a crystal axis of the nonlinear optical crystal even when the wavelength of inputted light changes.
To achieve the above-described object, according to a second aspect of the present invention, there is provided a sum frequency light generation apparatus comprising: a first polarization direction controller (5) which receives a fixed wavelength light (b1) having a single polarization direction and a fixed wavelength (xcex2) to control the polarization direction of the fixed wavelength light; a second polarization direction controller (2) which receives a variable wavelength light (a1) having a single polarization direction and a variable wavelength (xcex1) to control the polarization direction of the variable wavelength light so as to be perpendicular to the polarization direction of the fixed wavelength light; a nonlinear optical crystal (8) allowing type 2 phase matching which, when the fixed wavelength light whose polarization direction is controlled by the first polarization direction controller and the variable wavelength light whose polarization direction is controlled by the second polarization direction controller are entered, multiplexes the fixed wavelength light having an angular frequency xcfx89D and the variable wavelength light having an angular frequency xcfx89S, with the polarization directions thereof being perpendicular to each other so as to obtain a sum frequency light having an angular frequency xcfx89D+xcfx89S; and control means for, when multiplexing the fixed wavelength light and the variable wavelength light through the nonlinear optical crystal, controlling the polarization direction of the fixed wavelength so as to be parallel to a predetermined reference axis within a plane perpendicular to a phase matching direction (12) of the nonlinear optical crystal, wherein the predetermined reference axis is a single axis parallel to a crystal axis of the nonlinear optical crystal even when the wavelength of inputted light changes.
To achieve the above-described object, according to a third aspect of the present invention, there is provided a sum frequency light generation method in which a fixed wavelength light (b1) having a single polarization direction and a fixed wavelength (xcex2) and a variable wavelength light (a1) having a single polarization direction perpendicular to the fixed wavelength light and a variable wavelength (xcex2) are multiplexed on the same optical axis and the multiplexed fixed wavelength light and variable wavelength light are entered into a nonlinear optical crystal (8) allowing type 2 phase matching between the fixed wavelength light and the variable wavelength light, so that the nonlinear optical crystal emits a sum frequency light (c) of the fixed wavelength light and the variable wavelength light, comprising: with a first state in which the polarization direction of the variable wavelength light is set perpendicular to a predetermined reference axis in a plane substantially vertical to a phase matching direction (12) of the nonlinear optical crystal while the polarization direction of the fixed wavelength light is set parallel to the reference axis, detecting a first change rate of the phase matching direction of the nonlinear optical crystal when the wavelength of the variable wavelength light is changed; with a second state in which the polarization direction of the variable wavelength light is set parallel to the predetermined reference axis while the polarization direction of the fixed wavelength light is set perpendicular to the reference axis, detecting a second change rate of the phase matching direction of the nonlinear optical crystal when the wavelength of the variable wavelength light is changed; and setting each polarization direction of the fixed wavelength light and the variable wavelength light entered into the nonlinear optical crystal to each polarization direction set with the first state or the second state which detects a smaller change rate of the first change rate and the second change rate.
To achieve the above-described object, according to a fourth aspect of the present invention, there is provided a sum frequency light generation apparatus comprising: a multiplexer (3) which multiplexes an entered fixed wavelength light (b1) having a single polarization direction and a fixed wavelength (xcex2) and a variable wavelength light (a1) having a single polarization direction perpendicular to the polarization direction of the fixed wavelength light and a variable wavelength (xcex1) on the same optical axis (16); a nonlinear optical crystal (8) in which the fixed wavelength light and the variable wavelength light multiplexed by the multiplexer are entered and then a sum frequency light (c) of the fixed wavelength light and the variable wavelength light is emitted so as to allow type 2 phase matching between the fixed wavelength light and the variable wavelength light; and a polarization direction controller (5) which controls the polarization direction of the fixed wavelength light so as to be parallel to the reference axis of the nonlinear optical crystal while entering the fixed wavelength light into the nonlinear optical crystal.
To achieve the above-described object, according to a fifth aspect of the present invention, there is provided a sum frequency light generation apparatus comprising: a multiplexer (3) which multiplexes an entered fixed wavelength light (b1) having a single polarization direction and a fixed wavelength (xcex2) and a variable wavelength light (a1) having a single polarization direction perpendicular to the polarization direction of the fixed wavelength light and a variable wavelength (xcex1) on the same optical axis (16); a nonlinear optical crystal (8) in which the fixed wavelength light and the variable wavelength light multiplexed by the multiplexer are entered and then a sum frequency light (c) of the fixed wavelength light and the variable wavelength light is emitted so as to allow type 2 phase matching between the fixed wavelength light and the variable wavelength light; and a polarization direction controller (2, 5) which sets the polarization direction of the variable wavelength light to a direction perpendicular to a predetermined reference axis in a plane substantially vertical to a phase matching direction (12) of the nonlinear optical crystal while setting the polarization direction of the fixed wavelength light so as to be parallel to the predetermined reference axis, wherein the nonlinear optical crystal is 2-adamantyl amino-5-nitropyridine (AANP), which is an organic nonlinear optical crystal.
To achieve the above-described object, according to a sixth aspect of the present invention, there is provided a sum frequency light generation apparatus comprising: fixed wavelength light emitting means (20) for emitting a fixed wavelength light (b1) having a single polarization direction and a fixed wavelength (xcex2); a multiplexer (3) which receives the fixed wavelength light and variable wavelength light (a1) having a single polarization direction perpendicular to the polarization direction of the fixed wavelength light and a variable wavelength (xcex1) and multiplexes the fixed wavelength light and the variable wavelength light on the same optical axis (16); and a nonlinear optical crystal (8) in which the fixed wavelength light and variable wavelength light multiplexed by the multiplexer are entered and a sum frequency light (c) of the fixed wavelength light and the variable wavelength light is emitted so as to allow type 2 phase matching between the fixed wavelength light and the variable wavelength light, wherein the fixed wavelength light emitting means emits the fixed wavelength light so that the polarization direction of the emitted fixed wavelength light is parallel to a predetermined reference axis within a plane substantially vertical to a phase matching direction (12) of the nonlinear optical crystal.
To achieve the above-described object, according to a seventh aspect of the present invention, there is provided a sum frequency light generation apparatus according to the sixth aspect, wherein the nonlinear optical crystal is 2-adamantylamino-5-nitropyridine (AANP) which is an organic nonlinear optical crystal.
To achieve the above-described object, according to an eight aspect of the present invention, there is provided a sum frequency light generating apparatus according to the sixth aspect, wherein the fixed wavelength light emitting means is capable of emitting plural fixed wavelength lights each having a different wavelength and selectively emits any one of the plural fixed wavelength lights.
To achieve the above-described object, according to a ninth aspect of the present invention, there is provided a sum frequency light generation apparatus comprising: a multiplexer (3) which multiplexes an entered fixed wavelength light (b1) having a single polarization direction and a fixed wavelength (xcex2) and a variable wavelength light (a1) having a single polarization direction perpendicular to the polarization direction of the fixed wavelength light and a variable wavelength (xcex1) on the same optical axis (16); a nonlinear optical crystal (8) in which the fixed wavelength light and the variable wavelength light multiplexed by the multiplexer are entered and then a sum frequency light (c) of the fixed wavelength light and the variable wavelength light is emitted so as to allow type 2 phase matching between the fixed wavelength light and the variable wavelength light; incident angle changing means (30) for changing each incident angle of the fixed wavelength light and variable wavelength light into the nonlinear optical crystal; and a polarization direction controller (2, 5) which sets the polarization direction of the variable wavelength light so as to be perpendicular to a predetermined reference axis within a plane substantially vertical to a phase matching direction (12) of the nonlinear optical crystal while setting the polarization direction of the fixed wavelength light so as to be parallel to the predetermined reference axis.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.